The most widely used drug to combat pain is still morphine. There are severe effects of this drug, however, for prolonged use of morphine generally leads to physiological and psychological dependency on the drug. In addition, it has a depressing effect on respiration. A tireless and expensive search for an analgesic that would be as potent as morphine, but free of its dangerous effects, has been carried on. Many analgesic disclosures related to the morphine model have been synthesized, the best known of which is pethidine. This was originally thought to be a non-addicting drug, but was soon found to have dangerous addiction liability. Other synthetic analgesics include a group of substances called benzomorphans. The best known member of this group is phenazocine, but like morphine, this is also dangerously addicting.